Tolan derivative and a liquid crystal mixture containing the same

ABSTRACT

A novel tolan derivative compound as a liquid crystal substance having a low viscosity and a good compatibility with existing liquid crystals at low temperatures in addition to a large optical anisotropy value and a high clearing point, and a liquid crystal mixture containing the above compound are provided, which compound is expressed by the formula ##STR1## wherein R 1  represents an alkyl group of 1 to 10 carbon atoms, R 2  represents an alkyl group of 1 to 10 carbon atoms, X represents --CH 2  CH 2  -- and A represents an H atom or an F atom.

This application is a reissue of Ser. No. 07/079,997 filed Jul. 13,1987, U.S. Pat. No. 4,778,620.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a tolan derivative as a novel compound and aliquid crystal mixture containing the same.

2. Description of the Related Art

Display elements having liquid crystals applied thereto make use of theelectrooptical effect exhibited by liquid crystal substances, and thedisplay modes thereof include various ones such as those of TN (TwistedNematic) type, DS (Dynamic Scattering) type, guest-host type, DAP type,etc.

While the properties required for liquid crystal substances used varydepending on these respective modes, the following properties arerequired in common therewith:

a property that liquid crystal phases are exhibited within as broad atemperature range as possible and a property that the liquid crystalsubstances are stable to moisture, heat, light, air, etc. At present,however, there is no single compound which satisfies all of suchrequirements; thus there have been used liquid crystal mixtures obtainedby mixing several kinds of liquid crystal compounds or mixingcompound(s) similar to liquid crystal compounds with several kinds ofliquid crystal compounds.

As to tolan derivatives used as a component of liquid crystal materials,compounds expressed by the following formulas (1) to (3) are disclosedin (1) French patent application laid-open No. 2,141,438, (2) Japanesepatent application laid-open No. Sho 60-152427/1985 and (3) Japanesepatent application laid-open No. Sho 60-204731/1985, respectively:##STR2## wherein R³ and R⁴ each represent an alkyl group or an alkoxygroup. ##STR3## and R and R' each represent a linear chain alkyl group.

These tolan compounds are liquid crystal materials having specificfeatures of a large optical anisotropy value (hereinafter abbreviated toΔn) and a high clearing point.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a liquid crystalsubstance having a low viscosity and a good compatibility with existingliquid crystals at low temperatures in addition to the above-mentionedspecific features and also to enhance the degree of freeness of choiceof liquid crystal materials.

The present invention resides in a tolan derivative compound expressedby the formula ##STR4## wherein R¹ represents an alkyl group of 1 to 10carbon atoms, R² represents an alkyl group or an alkoxy group each of 1to 10 carbon atoms, X represents --CH₂ CH₂ -- or a single bond and Arepresents an H atom or an F atom provided that A represents an F atomin the case where X represents a single bond. The present invention isalso directed to a liquid crystal mixture comprising at least twocomponents at least one of which is said tolan derivative compound.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments of the present invention include compounds expressed bythe following group of formulas: ##STR5## wherein R¹ and R² are asdefined above.

These compounds are liquid crystalline compounds having a slightlynegative dielectric anisotropy value.

Preferred compounds among those expressed by the formula (I-a) are thosewherein R¹ represents ethyl, propyl, butyl or pentyl and R² representsmethyl, ethyl, propyl, butyl, pentyl, methoxy, ethoxy, propoxy, butoxyor pentyloxy in the formula (I-a).

For example, 4-(trans-4-propylcyclohexyl)-1-ethyl)-4'-ethyltolan shownin Example 1, described later, is a liquid crystal compound having alarge Δn value, a very low viscosity and a braod nematic range and itcan constitute a nematic liquid crystal material having various specificfeatures well balanced.

Preferred compounds among those expressed by the formula (I-b) are thosewherein R¹ represents ethyl, propyl, butyl or pentyl and R² representsethyl, propyl, butyl or pentyl in the formula. The tolan derivativesexpressed by the formula (1-a) or (1-b) are characterized in that anethylene group is introduced into a core structure having threesix-membered rings bonded linearly. It is presumed that by introducingthe ethylene group, a relatively low melting point (i.e. the lower limittemperature of a mesomorphic phase) and a low viscosity have beenrealized in the compounds of (I-a) and (I-b). Further, it is alsopresumed that as to be compounds of the formula (I-b), introduction of afluorine atom as a lateral substituent into the central phenylene ringof the compound of the formula (I-a) may have realized increase in thenematic range and decrease in the smectic range without increasing theviscosity so much as compared to the compound of the formula (I-a). Thecompound (I-b) is also a nematic liquid crystal having a large Δn valueand a low viscosity as in the case of the compound (I-a).

Preferred compounds among those of the formula (I-c) are those whereinR¹ or R² represents ethyl, propyl, butyl or pentyl. Some compounds ofthe formula (I-c), wherein both R¹ and R² have a long chain alkyl group,have a high viscosity so that such compounds are for practical reasonsnot particularly preferred. In the compounds of the formula (I-c) themelting point and the clearing point are lowered by a fluorinesubstitution at a lateral position of the central phenylene ring.Further, as to compounds expressed by the formulas (I-b) and (I-c), afluorine substitution at a lateral position of the central phenylenering improves the compatibility of the compound with other liquidcrystals at low temperatures.

The compounds of the present invention can be prepared for exampleaccording to the following preparation scheme, R¹, R², X and A thereinbeing as defined above: ##STR6##

A substituted benzene compound expressed by the formula (II) is firstreacted with a 4-substituted-phenylacetyl chloride and anhydrousaluminum chloride in carbon disulfide to obtain a ketone derivative ofthe formula (III), which is then reacted with a reducing agent such aslithium aluminum hydride in an anhydrous solvent such as ether ortetrahydrofuran to obtain a compound of the formula (IV). Successivelythis alcohol derivative is subjected to dehydration reaction in thepresence of a catalyst mentioned later in an inert organic solvent underatmospheric pressure and at a reflux temperature to obtain an ethylenederivative of the formula (V). As the inert organic solvent, benzene,toluene, chloroform, carbon tetrachloride, methylene chloride, etc. aresuitable, and as the catalyst, Lewis acids such as aluminum chloride,tin tetrachloride, titanium tetrachloride, etc., mineral acids such assulfuric acid, phosphoric acid, etc., toluenesulfonic acid, etc. areusable.

Successively bromine is attached onto the ethylene derivative of theformula (V) in a solvent such as methylene chloride, ethylene chloride,carbon tetrachloride, etc. to obtain a compound of the formula (VI),which is reacted with potassium tertiary-butoxide in tetrahydrofuransolvent, followed by a series of generally known purification proceduresof extraction, washing, recyrstallizatuon, etc. to obtain the objectivecompound of the formula (I). The reactions of the respective steps areknown, but the overall reactions are novel.

The substituted benzene of the formula (II) as the starting raw materialis obtained by applying a known reaction to an easily commerciallyavailable raw material. Namely, a substituted benzene of the formula(II) wherein X represents an ethylene group is obtained by reducing aketone derivative obtained by a Friedel-Crafts reaction of atrans-4-alkylcyclohexylacetyl chloride with benzene or by couplingreaction of a trans-4-alkylcyclohexylacetyl chloride withbis(3-fluorophenyl)cadmium, while a substituted benzene of the formula(II) wherein X represents a single bond is obtained by subjecting asubstituted cyclohexanol obtained by a Grignard reaction of3-fluorophenylmagnesium bromide with a 4-alkylcyclohexanone, to adehydration reaction and then a hydrogenation reaction.

Preferred representative examples of liquid crystal compounds used ascomponents of the liquid crystal mixture of the present inventionadmixture with the compound expressed by the formula (I) are

4-substituted-phenyl 4'-substituted-benzoates,

4-substituted-phenyl 4'-substituted-cyclohexanecarboxylates,

4'-substituted-biphenyl-4-yl 4"-substituted-cyclohexanecarboxylates,

4-substituted-phenyl 4'-(4-substituted-cyclohexanecarbonyloxy)benzoates,

4-substituted-phenyl 4'-(4-substituted-cyclohexyl)benzoates,

4-substituted-cyclohexyl 4'-(4-substituted-cyclohexyl)benzoates,

4,4'-substituted-biphenyls,

4,4'-substituted-phenylcyclohexanes,

4,4"-substituted-terphenyls,

4,4"-substituted-biphenyl-4'-yl-cyclohexanes,

2-(4'-substituted-phenyl)-5-substituted-pyrimidines, etc.

Compounds having lateral substituent(s) of halogen atom(s) or cyanogroup(s) on the phenylene ring(s) of the above-mentioned compounds mayalso be used as components of the liquid crystal mixture of the presentinvention.

As is generally known, in a liquid crystal display element, the productof the cell thickness (d) and the optical anisotropy value (Δn) of theliquid crystal material employed must be set to a specified value inorder to prevent an interference fringe on the cell surface whichdeteriorates the view of the display. In practice, the value of Δn×d hasbeen set to any one of 0.5, 1.0, 1.6 or 2.2. Since the value of Δn×d isset to definite values as described above, if a liquid crystal materialhaving a large Δn value is used, it is possible to reduce the d value.When the d value is reduced, the response time is reduced. Thus, liquidcrystal materials having a large Δn value are important for preparing aliquid crystal display cell having a high response rate and nointerference fringe. Further, in order to reduce the response time, alow viscosity is also necessary.

The compounds corresponding to formula (I) of the present invention arenovel nematic liquid crystal compounds having a large Δn value (ca.0.26), a high N-I transition point and further a low viscosity (ca. 16cp at 20° C.). Thus, when a compound of the formula (I) of the presentinvention is mixed with various mother liquid crystals, it is possibleto prepare a practical liquid crystal material having a low viscosity, alarge Δn value and a high N-I transition point.

The present invention will be described more concretely by way ofexamples, but it should not be construed to be limited thereto.

In addition, symbols C-S point, C-N point, S-N point and N-I point inthe examples refer to crystalline-smectic transition point,crystalline-nematic transition point, smectic-nematic transition pointand nematic-isotropic liquid transition point, respectively.

EXAMPLE 1 4-(Trans-4-propylcyclohexyl-1-ethyl)-4'-ethyltolan

Anhydrous aluminum chloride (27.7 g, 0.21 mol) was added to carbondisulfide (160 cc), followed by adding 4-ethylphenylacetyl chloride(31.7 g, 0.174 mol) under cooling, successively addingtrans-4-propylcyclohexylethylbenzene (40 g, 0.174 mol) over about onehour, thereafter agitating the reaction mixture at room temperature for10 hours, distilling off carbon disulfide, adding the residue to dilutehydrochloric acid aqueous solution, decomposing the resulting aluminumchloride complex with stirring for one hour, extracting the resultingraw crystals with toluene (100 cc), washing with water, drying,distilling off toluene and recrystallizing the remaining solids fromethyl acetate to obtain the following compounds (35.3 g): ##STR7##

This compound was dissolved in anhydrous tetrahydrofuran (50 cc),followed by dropwise adding the solution to a solution of lithiumaluminum hydride (2.7 g, 0.07 mol) in anhydrous tetrahydrofuran (100cc), further agitating the mixture at 0° C. for one hour, thereafteradding 20 wt. % sulfuric acid (50 cc) to the reaction mixture todissolve the inorganic substance, extracting the separated oilysubstance with toluene (100 cc), washing the separated toluene solutionwith 10% sodium hydrogen carbonate aqueous solution, further washingwith water until the washing water became neutral, drying the resultingtoluene solution with anhydrous sodium sulfate, adding p-toluenesulfonicacid (1.8 g), heating the mixture under reflux, removing the formedwater to the outside of the system, allowing the resulting material tocool down to room temperature after completion of the reaction, washingthe toluene solution with water until the washing water became neutral,drying the toluene solution with anhydrous sodium sulfate, distillingoff toluene and recrystallizing the remaining solids from ethyl acetateto obtain the following compound (25 g): ##STR8##

This product exhibited liquid crystal phases and the transition pointswere as follows: C-S point: 143.9° C., S-N point: 151.3° C. and N-Ipoint: 197.9° C.

This compound was dissolved in methylene chloride (150 cc), followed bydropwise addition of bromine (11.0 g, 0.069 mol) at room temperature andreaction of the mixture for one hour, thereafter distilling offmethylene chloride and recrystallizing the remaining solids fromn-heptane to obtain the following compound (24.5 g): ##STR9##

This compound was then dissolved in anhydrous tetrahydrofuran (100 cc),followed by adding potassium tertiary-butoxide (22.5 g, 0.20 mol),agitating the mixture at 40° C. for 2 hours, thereafter adding water(200 cc) to the reaction mixture, extracting the separated raw crystalswith toluene (100 cc), washing with water, drying, distilling offtoluene and recrystallizing the remaining solids from ethyl acetate toobtain the following compound (15.0 g): ##STR10##

This compound exhibited liquid crystal phases and the transition pointswere as follows: C-S point: 68.7° C., S-N point: 74.1° C. and N-I point:161.8° C.

EXAMPLES 2 TO 16

Tolan derivatives obtained in the same manner as in Example 1 and theirphase transition points are shown in Table 1 together with the resultsof Example 1.

                  TABLE 1                                                         ______________________________________                                         ##STR11##                                                                                  Phase transition point (°C.)                                                     C-S point                                                                     or                                                    Example                                                                              R.sup.1  R.sup.2 C-N point                                                                             S-N point                                                                            N-I point                              ______________________________________                                        1      n-C.sub.3 H.sub.7                                                                      C.sub.2 H.sub.5                                                                       68.7    74.1   161.8                                  2      C.sub.2 H.sub.5                                                                        CH.sub.3                                                                              84.3           144.1                                  3      C.sub.2 H.sub.5                                                                        C.sub.2 H.sub.5                                                                       69.4           141.2                                  4      C.sub.2 H.sub.5                                                                        n-C.sub.3 H.sub.7                                                                     66.1           149.2                                  5      C.sub.2 H.sub.5                                                                        n-C.sub.4 H.sub.9                                                                     59.1    62.3   137.4                                  6      n-C.sub.3 H.sub.7                                                                      CH.sub.3                                                                              83.6           169.1                                  7      n-C.sub.3 H.sub.7                                                                      n-C.sub.3 H.sub.7                                                                     67.1    77.6   166.6                                  8      n-C.sub.3 H.sub.7                                                                      n-C.sub.4 H.sub.9                                                                     36.4    90.0   158.8                                  9      n-C.sub.4 H.sub.9                                                                      CH.sub.3                                                                              85.6    (77.8) 162.3                                  10     n-C.sub.4 H.sub.9                                                                      C.sub.2 H.sub.5                                                                       69.5    92.7   155.7                                  11     n-C.sub.4 H.sub.9                                                                      n-C.sub.3 H.sub.7                                                                     65.6    94.2   161.6                                  12     n-C.sub.4 H.sub.9                                                                      n-C.sub.4 H.sub.9                                                                     59.8    103.1  152.5                                  13     n-C.sub.5 H.sub.11                                                                     CH.sub.3                                                                              91.7    (85.6) 165.3                                  14     n-C.sub.5 H.sub.11                                                                     C.sub.2 H.sub.5                                                                       81.5    100.4  158.0                                  15     n-C.sub.5 H.sub.11                                                                     n-C.sub.3 H.sub.7                                                                     67.9    112.1  165.4                                  16     n-C.sub.5 H.sub.11                                                                     n-C.sub.4 H.sub.9                                                                     55.2    119.9  156.3                                  ______________________________________                                    

In Table 1 and the succeeding Tables 2 and 3, the symbol () indicates amonotropic transition point.

EXAMPLES 17 TO 25

Compounds shown in Table 2 were obtained from3-(trans-4-alkylcyclohexylethyl)fluorobenzenes and4-substituted-phenylacetyl chlorides in the same manner as in Example 1.

                  TABLE 2                                                         ______________________________________                                         ##STR12##                                                                                  Phase transition point (°C.)                                                     C-S point                                                                     or                                                    Example                                                                              R.sup.1  R.sup.2 C-N point                                                                             S-N point                                                                            N-I point                              ______________________________________                                        17     C.sub.2 H.sub.5                                                                        n-C.sub.3 H.sub.7                                                                     36.0           135.2                                  18     C.sub.2 H.sub.5                                                                        n-C.sub.4 H.sub.9                                                                     26.1           126.5                                  19     n-C.sub.3 H.sub.7                                                                      CH.sub.3                                                                              73.2           158.3                                  20     n-C.sub.3 H.sub.7                                                                      C.sub.2 H.sub.5                                                                       50.8           149.8                                  21     n-C.sub.3 H.sub.7                                                                      n-C.sub.3 H.sub.7                                                                     50.2           155.5                                  22     n-C.sub.3 H.sub.7                                                                      n-C.sub.4 H.sub.9                                                                     42.0           148.6                                  23     n-C.sub.4 H.sub.9                                                                      C.sub.2 H.sub.5                                                                       58.3           147.8                                  24     n-C.sub.4 H.sub.9                                                                      n-C.sub.3 H.sub.7                                                                     56.5    (43.5) 155.1                                  25     n-C.sub.4 H.sub.9                                                                      n-C.sub.4 H.sub.9                                                                     50.4    71.8   147.1                                  ______________________________________                                    

EXAMPLES 26 TO 32

Compounds shown in Table 3 were obtained from3-(trans-4-alkylcyclohexyl)fluorobenzenes and 4-substituted-phenylacetylchlorides in the same manner as in Example 1.

                  TABLE 3                                                         ______________________________________                                         ##STR13##                                                                                     Phase transition                                                              point (°C.)                                           Example  R.sup.1   R.sup.2 S-N point                                                                             N-I point                                  ______________________________________                                        26       n-C.sub.3 H.sub.7                                                                       CH.sub.3                                                                              103.5   198.0                                      27       n-C.sub.3 H.sub.7                                                                       C.sub.2 H.sub.5                                                                       78.3    191.0                                      28       n-C.sub.3 H.sub.7                                                                       n-C.sub.3 H.sub.7                                                                     79.1    199.3                                      29       n-C.sub.3 H.sub.7                                                                       n-C.sub.4 H.sub.9                                                                     50.5    187.7                                      30       C.sub.2 H.sub.5                                                                         C.sub.2 H.sub.5                                                                       50.6    166.9                                      31       C.sub.2 H.sub.5                                                                         n-C.sub.3 H.sub.7                                                                     49.8    175.8                                      32       C.sub.2 H.sub.5                                                                         n-C.sub.4 H.sub.9                                                                     49.3    162.1                                      ______________________________________                                    

EXAMPLE 33

A liquid crystal mixture A consisting of

    ______________________________________                                                   trans-4-propyl-(4-cyanophenyl)                                                                     30% by weight.                                           cyclohexane                                                                   trans-4-pentyl-(4-cyanophenyl)                                                                     40% by weight.                                           cyclohexane                                                            and                                                                                  trans-4-heptyl-(4-cyanophenyl)                                                30% by weight                                                                 cyclohexane                                                        ______________________________________                                    

has a N-I point of 52.1° C., a viscosity of 20° C. of 22.4 cp and anoptical anisotropy value Δn of 0.119.

When 4-(trans-4-propylcyclohexyl-1-ethyl)-4'-ethyltolan (15 parts byweight) as a compound of the present invention shown in Example 1 wasadded to the liquid crystal mixture A (85 parts by weight), the N-Ipoint of the resulting liquid crystal mixture rose to 68.0° C., theviscosity at 20° C. thereof lowered to 19.0 cp and the opticalanisotropy value Δn thereof rose to 0.142.

Further, even when this liquid crystal mixture was allowed to stand at-30° C. for 30 days, no deposition of crystal occurred.

EXAMPLE 34

When 2-fluoro-4-(trans-4-propylcyclohexyl-1-ethyl)-4'-ethyltolan (15parts by weight) as a compound of the present invention shown in Example20 was added to the liquid crystal mixture A (85 parts by weight) usedin Example 33, the N-I point of the resulting liquid crystal mixturerose to 65.0° C., the viscosity at 20° C. thereof lowered to 20.8 cp andthe optical anisotropy value Δn rose to 0.140.

Further, even when this liquid crystal mixture was allowed to stand at-30° C. for 30 days, no deposition of crystal occurred.

EXAMPLE 35

When 2-fluoro-4-(trans-4-propylcyclohexyl)-4'-ethyltolan (15 parts byweight) as a compound of the present invention shown in Example 27 wasadded to the liquid crystal mixture A (85 parts by weight) used inExample 33, the N-I point of the resulting liquid crystal mixture roseto 66.7° C., the viscosity at 20° C. lowered to 21.0 cp and the opticalanisotropy value Δn rose to 0.140.

Further, even when this liquid crystal mixture was allowed to stand at-30° C. for 30 days, no deposition of crystal occurred.

It can be seen from these Examples that the compound of the formula (I)of the present invention can raise the N-I point of the mother liquidcrystals up to a practically sufficient value, can raise the Δn thereofand yet can reduce the viscosity thereof and also is a liquid crystalmaterial having a superior compatibility at low temperatures.

COMPARATIVE EXAMPLE 1

When 4-(trans-4-propylcyclohexyl)-4'-ethyltolan (15 parts by weight) asone of tolan derivatives shown in the formula (2) as a prior art wasadded to the liquid crystal mixture A (85 parts by weight) used inExample 33, the N-I point of the resulting liquid crystal mixture roseto 68.3° C., the viscosity at 20° C. thereof was 22.8 cp and the opticalanisotropy value thereof rose to 0.141, but when the mixture was allowedto stand at -30° C., deposition of crystal occurred in only one day.

COMPARATIVE EXAMPLE 2

When 4-(4-propylphenyl)-4'-ethyltolan (15 parts by weight) as one oftolan derivatives shown in the formula (2) as a prior art was added tothe liquid crystal mixture A (85 parts by weight) used in Example 33,the N-I point of the resulting liquid crystal mixture rose to 70.3° C.and the Δn thereof rose to 0.145, but the viscosity at 20° C. thereofalso rose to 28.0 cp. When this liquid crystal mixture was allowed tostand at -30° C., deposition of crystal occurred in only 5 hours.

What we claim is:
 1. A tolan derivative compound expressed by theformula ##STR14## wherein R¹ represents an alkyl group of 1 to 10 carbonatoms, R² represents an alkyl group of 1 to 10 carbon atoms, Xrepresents --CH₂ CH₂ -- and A represents an H atom or an F atom.
 2. Atolan derivative compound according to claim 1 wherein A represents H.3. A tolan derivative compound according to claim 1 wherein said Xrepresents --CH₂ CH₂ -- and said A represents F.
 4. A liquid crystalmixture comprising at least two components at least one of which is atolan derivative compound expressed by the formula ##STR15## wherein R¹represents an alkyl group of 1 to 10 carbon atoms, R² represents analkyl group of 1 to 10 carbon atoms, X represents --CH₂ CH₂ -- and Arepresents an H atom or an F atom. .Iadd.
 5. A tolan derivative compoundexpressed by the formula ##STR16##.Iaddend. wherein each of R¹ and R²independently represents an alkyl group of 2 to 5 carbon atoms. .Iadd.6. A tolan derivative compound according to claim 5 wherein R¹ and R²are the same. .Iaddend. .Iadd.7. A tolan derivative compound accordingto claim 5 wherein R¹ and R² each comprise an ethyl group. .Iaddend..Iadd.8. A tolan derivative compound according to claim 5 wherein R¹ andR² each comprise a propyl group. .Iaddend. .Iadd.9. A tolan derivativecompound according to claim 5 wherein R¹ comprises an ethyl group and R²comprises a propyl group. .Iaddend. .Iadd.10. A tolan derivativecompound according to claim 5 wherein R¹ comprises an ethyl group and R²comprises a butyl group. .Iaddend. .Iadd.11. A tolan derivative compoundaccording to claim 5 wherein R¹ comprises a propyl group and R²comprises an ethyl group. .Iaddend. .Iadd.12. A tolan derivativecompound according to claim 5 wherein R¹ comprises a propyl group and R²comprises a butyl group. .Iaddend. .Iadd.13. A liquid crystal mixturecomprising at least two components, at least one of which is a tolanderivative compound expressed by the formula ##STR17##.Iaddend. whereineach of R¹ and R² independently represents an alkyl group of 2 to 5carbon atoms. .Iadd.14. A liquid crystal mixture according to claim 13wherein R¹ and R² are the same. .Iaddend. .Iadd.15. A liquid crystalmixture according to claim 13 wherein R¹ and R² each comprise an ethylgroup. .Iaddend. .Iadd.16. A liquid crystal mixture according to claim13 wherein R¹ and R² each comprise a propyl group. .Iaddend. .Iadd.17. Aliquid crystal mixture according to claim 13 wherein R¹ comprises anethyl group and R² comprises a propyl group. .Iaddend. .Iadd.18. Aliquid crystal mixture according to claim 13 wherein R¹ comprises anethyl group and R² comprises a butyl group. .Iaddend. .Iadd.19. A liquidcrystal mixture according to claim 13 wherein R¹ comprises a propylgroup and R² comprises an ethyl group. .Iaddend. .Iadd.20. A liquidcrystal mixture according to claim 13 wherein R¹ comprises a propylgroup and R² comprises a butyl group. .Iaddend.